Various hybrid electric vehicles are proposed to reduce emission of carbon dioxides (CO2) of an internal combustion engine for the purpose of environment protection. As a motive power source of the hybrid electric vehicle, an AC motor and a high voltage main power source (DC power source) are mounted on the vehicle in addition to the engine. For improving fuel efficiency, it is required to further improve drive efficiency of the AC motor and reduce size of the AC motor
To meet these requirements, JP 8-214592A (patent document 1) proposes to chopper-boost a voltage of a DC power source to generate a boosted system voltage, convert this DC system voltage into an AC voltage by an inverter and drive the AC motor with the AC voltage. JP 2005-45880A (patent document 2) proposes to switch over a control mode of an AC motor among a sine-wave PWM control mode, a rectangular-wave control mode and an over-modulation PWM mode in accordance with an operating condition of the AC motor. JP 2007-306658A (patent document 3) proposes to vary a target value of a system voltage boosted by a boosting converter in accordance with a control mode of an AC motor. In the patent document 3, the boosting converter is controlled so that the system voltage produced actually attains the target value, and an inverter is controlled to adjust an AC voltage supplied to the AC motor so that an output torque of the AC motor produce actually attains a target torque.
In the AC motor drive systems proposed by the patent documents 1 to 3, in which the voltage of the DC power source is boosted by the boosting converter and the boosted system voltage is converted into the AC voltage, a current flowing in the AC motor varies and hence the output torque of the AC motor varies, when the system voltage varies and the AC voltage supplied to the AC motor varies.
In the systems, which perform both voltage control and torque control, the boosting converter is controlled to attain the target system voltage by the voltage control and the inverter is controlled to attain the target output torque of the AC motor. The variation in the system voltage caused by the voltage control influences the output torque of the AC motor and the variation in the output torque (variation in the AC voltage) of the AC motor caused by the torque control influences the system voltage. That is, the voltage control and the torque control interfere each other, thus lowering performance of the voltage control and the torque control and causing variations in the system voltage and the output torque.
If the system voltage on a system power supply line varies and reaches an over-voltage, electronic devices connected to the system power line may be broken due to the over-voltage.
To suppress generation of an excessive voltage, WO 2007/069413A1 corresponding to JP 2007-166875A (patent document 4) proposes to reduce a target value of a system voltage in a case of rectangular-wave control or over-modulation PWM control of an AC motor to be lower than in a case of sine-wave PWM control.
According to the patent document 4, the system voltage is reduced in the case of rectangular-wave control or over-modulation PWM control of the AC motor to be lower than in the case of sine-wave PWM control to suppress the over-voltage. However, this control reduces the target voltage of the system voltage uniformly in the rectangular-wave control. Therefore, the system voltage cannot be used effectively and hence the performance of the AC motor will be lowered.